This invention relates to orthopedic and prosthetic implantable devices adapted for bone ingrowth and high strength, as well as using metal injection molding with sacrificial monolithic inserts that define the surface of at least a portion of the devices.
In the field of orthopedic devices and prostheses, coatings have been used to provide an ingrowth medium and texture to enable initial and long-term fixation of the devices to the bone. These implantable devices are generally bone prostheses and are known to include components of artificial joints, such as hips, knees, elbows and shoulders. The strength of coatings on such implants is an important consideration for short term and long term fixation to the bone.
Known methods of manufacturing coatings for implants include formation by foaming, plasma spraying, diffusion bonding or sintering of powders, depositing a material upon a substrate, use of vaporizing materials, and chemical and plasma etching. Metal injection molding has found only limited application, if any, in the manufacture of implantable devices for many reasons, including the complexities of the coating surface and ingrowth geometries, as well as the strength requirements of the devices. The fact that a molded part has to release from the mold limits the geometries of the surface. If the molded implant has an undercut, it cannot be simply separated from the mold.
It is often desirable to have features on an orthopedic or prosthetic device such as complex textures, undercuts, porosities, cavities, and other texture geometries that would not release from a mold if the device is manufactured by metal injection molding. These surfaces are intended to offer initial fixation, an ingrowth surface, and other properties desirable for implants.
Texturing the mold used for metal injection molding can achieve a roughened surface on a molded implant. The mold surface would be created as a negative of the desired texture, so as to impart the positive of the desired texture to the article during molding. Applying this methodology to more complex shapes or textures has fundamental challenges.
A complex shape or surface would include sections that have undercuts, undercut texture, internal porosity and details or elements obscured from the external surface. As such, a complex surface would not release from a mold due to its geometry.
While it may be possible to apply a simple texture to a flat surface in a mold, it is often desired to apply texture continuously around the perimeter or circumference of a part such as hip stem or acetabular cup used as an implant. Depending on the nature of the desired geometry, mold texturing has significant limitations for the nature of the textured surface imparted on the device. For example, texture around the circumference of an implant will create internal features that prevent the molded part from releasing from the mold. In some cases this can be partially addressed with complicated tooling, but the geometry of the ingrowth surface is still limited.
Even on flat surfaces, mold texturing can only create surfaces that can be released from the mold. Again, this limits the complexity of the textured portion. However, a textured portion having a complex geometry is important because it allows the surface to have “grippiness” in multiple directions.
Expendable or sacrificial inserts have been used in the mold to create tooling surfaces that form a surface geometry. Such insert can then be removed post-molding via dissolution, decomposition or other methods. However, there remains a challenge of creating the insert with appropriate complex negative texture since undercuts and other complex geometries need to be formed in the insert. Devices molded with insert fail to be able to define the entire complex character of surface and ingrowth region of the device. Moreover, the strength requirements of implants limit the complexities of such surfaces as stress concentrators form during injection molding that limit the ultimate performance of the device.
An insert for metal injection molding of a textured portion must have the negative geometry of the desired textured portion. Due to the strength requirements and complexities of implant devices, the textured portion frequently cannot be created in the insert by traditional manufacturing routes. For example, machining is a line-of-sight, subtractive manufacturing process that cannot create the complexities of a fixation texture, such as undercuts, internal cavities for ingrowth, or the substrate interfaces on the same insert. The ingrowth medium and substrate interface are obscured from the machining tool by both the ingrowth medium and the fixation texture.
Furthermore, porous coatings applied to implantable devices tend to yield reduced fatigue strength resulting from stress concentrations that arise from the interface between the coating and the dense substrate. The addition of osseo-integration surface coatings to implantable bodies diminishes the fatigue strength due to the surface interruptions at the interface between the ingrowth medium and the dense substrate. These interruptions create stress concentrations that in turn can serve as fracture initiation sites, significantly reducing the fatigue life of the device. Unfortunately, titanium, the preferred material for device bodies due to its biocompatibility, is especially vulnerable to fatigue life reduction due to perturbations on the surface. Stress concentrations at the substrate interface are a common problem with device coatings.
Additionally, the metal injection molding of titanium for implantable device surfaces using inserts brings with it several unique challenges relative to conventional metal injection molding. The issue of primary concern is contamination due to inadequate insert removal or reaction of the insert material with the titanium powder or binder during processing leading to reduced tensile bond strength of the device.
There remains a need to create a high strength implant device by metal injection molding that is at least partially porous. In addition, a device having a specified surface texture for an injection molded article while having high tensile strength and appropriate fatigue performance is sought. It would be desirable, to have a method of tailoring textured portions, including undercuts, porosity, surface curvature, and other features for injection molded implants.